This invention relates to hydrocarbon recovery systems, and more specifically, to systems for recovering floating hydrocarbons such as gasoline or oil from wells.
In the recent past there has been an increased public awareness of the effects of pollution in our environment. This has lead to a great deal of legislation being passed to clean up pollution. One area of our environment that is being polluted is groundwater through the leakage of piping and tanks containing petroleum byproducts (e.g., oil). Water will dissolve only small amounts of oil. Therefore, if oil is present in significant amounts, it will form a separate phase, forming a pool of oil on top of the water table. When this is detected, the law generally mandates that a program to remove the oil be initiated immediately, followed by a program to remediate the contaminated groundwater.
A common method of removing the oil is to hand bail the well. This is neither practical nor cost effective on an ongoing basis.
Another method of removing the oil is to position a pump in such a manner that it draws liquid from near the top of the water table. This is known as a total fluids pumping system. The theory is that the oil recovery rate can be increased by drawing down the water table into a "cone of depression". The oil then flows down the surface of this cone of depression into the pump inlet. This method requires the installation of expensive above ground oil/water separation equipment and subsequent treatment of large quantities of water before it can be discharged. Another problem with this method, is that when the pump(s) are periodically turned off, the water table recovers to it's original level. This "smears" the oil through the soil and further contaminates the groundwater. This method does remove the contaminated groundwater, but the presence of the free phase continues to contaminate the groundwater as it is being pumped.
Yet another method of removing the oil is by using a method referred to as a specific gravity skimmer. This method calls for the placement of a pump with a floating inlet down the well. The specific gravity of the float is greater than oil and less than water. Therefore, the inlet is just above the water. This method is typically capable of removing the oil down to a thickness of from about one-half to about one-quarter inch. The float can then be weighted to be just below the oil/water interface and pump both oil and water with the same problems as the total fluids system mentioned above.
One potential problem with skimming systems is the donut effect theory. This theory states that if the oil layer is thick enough, the water table will be depressed due to the weight of the oil. If the oil in the well is pumped out, the water table near the well will rebound. Since the specific gravity of oil is typically about 0.75, the water table will rebound by an amount equal to about three-quarters the thickness of the oil layer. This local rebound is referred to as the donut and slows the recovery of oil. The way around this is to pump total fluids or to use a two pump system. In the two pump system, a second groundwater pump is placed below the skimmer to depress the water table and enhance the oil recovery. This allows down well oil/water separation. It also requires a water treatment system and often a larger well.